Popped a 25 amp fuse with 260 watts of solar?

I'm set up in the lab (my patio) with my new 12 volt off grid system. I have a new Midnite 150 lite, 2 GC2's in series, 2 130 watt pannels in series. I had the same battery bank and series wired pannels on my sunsaver mppt with 15 amp fuses and never popped one. Now as before my pannels run through an inline 15 amp fuse and my loads run through another one. The battery bank is connected through a 25 amp inline fuse that popped yesterday on a cloudy day. The clouds never broke so direct sun never happend but they did lighten slightly and I saw 10 amps of charge current for an hour or so then pop. When I got home the clouds were thick again and I replaced the fuse with a 15 amp one just to be extra cautious and the system ran all night just fine. I've powered down this morning. Could it have been a sweep?

- Jeff

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That would be an Edge Of Cloud event; just the right (wrong) conditions and the sun hiding behind the clouds comes through an edge of the moisture which focuses the sunlight momentarily causing the panel current to shoot up. Under such conditions you can get a real 260+ Watts from the panels. Divide by 12 and you get over 20 Amps. The controller won't react fast enough to clip this.

25 Amp is actually the right fuse size. Edge-of-cloud events are why you keep spares.

My third pannel arrived a few days ago. My baby box with Midnite DC din rail breakers is here as well. I got a 15 amp for the solar array, and a 40 amp for the battery. Will I pop the 40 amp with 390 watts of solar? My pannels will be in series with STC of 7.3 amps at 54 volts.

The Midnite is capable of something like 90 amps of output current... One way to avoid popped breakers is to reprogram the Midnite controller to output (I think the max output current is programmable):

40 amps * 1/1.25 NEC derating = 32 amps

That will ensure that the breaker never trips by accident. (breakers are rated to trip >80% of rated current--but it may take an hour or more to trip at 100% of rated current--NEC is already pretty conservative, so you could probably get away with 40 amp as the max current--But this is between you and your controller if you wish to do this--Part if it depends on what happens if the breaker trips--Will you be there to simply turn it back on, or will the battery bank go dead from unattended loads--the old engineering system questions at affect reliability and safety).

I'm not sure that even a MidNite Classic can clamp an E-O-C event fast enough to prevent a sudden current surge. Since the circuit protection on a controller's output is there to protect that circuit, wiring and fusing for maximum expected output current (regardless of anticipated operating parameters) will minimize the risk.

Not allowing the battery to go too low helps also: 260 Watts at 11 Volts is 23.6 Amps whereas at 12 it's 21.6 - 2 Amps can make the difference between blow/no blow. (This is looking at the battery as a resistance load when charging; lower Voltage = lower resistance = higher current.)

But if you look at Bill's math you can see the panels could possible pop 50 Amps.
The question is, how often will this happen and how devastating will it be if it does?

It will not hurt to wire it up for higher current as the controller can take it, it just won't produce it. The main reason there's circuit protection on the output is to stop fires if the controller should short because the battery can then push huge amounts of current through the short. Fuse blows/breaker trips to stop that current before the fire starts. As long as the circuit protection is lower current than the wiring and the current potential (in this case from the battery to the controller; handling it the other way is easy) it will work. The panels can't output more than the controller will pass, even with "too much" panel on the input.

The Midnite is capable of something like 90 amps of output current... One way to avoid popped breakers is to reprogram the Midnite controller to output (I think the max output current is programmable):

40 amps * 1/1.25 NEC derating = 32 amps

That will ensure that the breaker never trips by accident. (breakers are rated to trip >80% of rated current--but it may take an hour or more to trip at 100% of rated current--NEC is already pretty conservative, so you could probably get away with 40 amp as the max current--But this is between you and your controller if you wish to do this--Part if it depends on what happens if the breaker trips--Will you be there to simply turn it back on, or will the battery bank go dead from unattended loads--the old engineering system questions at affect reliability and safety).

-Bill

There is in fact an output current setting that is set to 96 amps. I've asked if I can adjust this. I've also read everything I can find and found no information on this topic. Maybe I've read too much and its all a blur.

Thank you for the math. According to your calculations I need a 60 amp breaker for the battery connection and if possible limit the output current to 45 amps or so?

There is in fact an output current setting that is set to 96 amps. I've asked if I can adjust this. I've also read everything I can find and found no information on this topic. Maybe I've read too much and its all a blur.

Thank you for the math. According to your calculations I need a 60 amp breaker for the battery connection and if possible limit the output current to 45 amps or so?

I know that the midnite Classic 150 can be programmed to limit the current output to the batteries and is adjustable in 1 amp increments. The midnite Classic lite, can also be programmed, but because there is no built in display, must be done externally. This link explains the methods that can be used to program the lite version, which is otherwise functionally equivalent to the 150.

I got the answer on limiting output. No problem, it will limit the output by drawing less from the array just like it does in float. I've set the output down to 24 amps for now due to the 10 awg wiring in my "lab" and not wanting to over charge the 225ah battery bank. I have a steady 0.8 amp load so a possible 23 amps going to the battery's. This is somewhere close to C13 charge rate I believe.

One of the good things about being able to limit current output (as found on the better controllers in black boxes ) is that you can oversize the array for charging on bad days and not have a problem when the sun does shine. Not all charge controllers have this ability.

One of the good things about being able to limit current output (as found on the better controllers in black boxes ) is that you can oversize the array for charging on bad days and not have a problem when the sun does shine. Not all charge controllers have this ability.